This invention relates to the field of force measurement. More particularly, this invention relates to a method and apparatus for measuring the residual tension in a bolt or stud of a previously tightened joint.
In many situations it is highly desirable to measure the residual tension in bolts or studs in a joint system. There are many instruments for imposing and measuring predetermined loads on bolts or studs of a joint when the joint is initially being assembled. These instruments include various kinds of torque wrenches with force measuring read outs such as shown in U.S. Pat. No. 3,683,686, or tensioners. Torque wrenches operate by imposing a torque load on a nut or bolt head to tighten a joint. Tensioners, such as those available from Biach Industries Inc. of Cranford, New Jersey as shown in U.S. Pat. No. 3,995,828 and other patents cited therein, operate by pulling on a bolt or stud to elongate the stud proportional to a desired tension load. The nut is then run down to butt against the flange or plate of the joint, and the tensioner is then removed.
Regardless of the way in which a joint is originally loaded, bolts or studs may lose tension over a period of time as the result of various effects, such as vibration, heat, or radiation. Since it is known that a bolt or stud will elongate in proportion to the tension load imposed on it, it is sometimes possible to obtain an indication of changes in tension by measuring the change in length of the bolt or stud. However, in many situations bolts and studs lose tension without any accompanying change in length. This is particularly so when bolts or studs are subjected to heat and/or nuclear radiation. Thus, there is a particular and important need to be able to determine the residual tension in bolts and studs in nuclear apparatus where the bolts and studs may lose tension without change in length as the result of exposure to heat and/or radiation. Unless residual tension can be determined, the integrity of the joint may be an unknown factor, a situation which could result in serious consequences in installations such as nuclear reactors.